How did measurements come into place?

I am actually curious to this measurements thing.
How do you know that a certain distance of 1 cm is 1 cm? I will be a bit clearer, we all know how long is 1 cm right but it strike me to ask how did people know that the certain length or distance is 1 cm? Who came up with these measurements idea? This concept applies to every other units such as m,km,nm and all those units. How do you know for sure 100 cm is 1m?

Historically, the metre was defined by the French Academy of Sciences as the length between two marks on a platinum-iridium bar, which was designed to represent 1⁄10,000,000 of the distance from the equator to the north pole through Paris. In 1983, it was redefined by the International Bureau of Weights and Measures (BIPM) as the distance travelled by light in free space in 1⁄299,792,458 of a second. The BIPM does not distinguish between quantum vacuum and free space.

My guess would be that at some point in history, some people that liked to build things got together and decided on universal measurements. They had to come up with some kind of constant so that things would be easier. The units that they decided on were completely their own creation but it's stuck around since then.

As measurements are a man made creation, it makes it easier to realize that any distance can really be that distance because someone said so.

The cubit of Noah's time was the length of a man's forearm or the distance from the tip of the elbow to the end of his middle finger. Many times this was useful, because it was readily available, convenient, and couldn't be mislaid. However, it was not a positive fixed dimension or a standard.
While the cubit is no longer used as a unit of measurement, there are many customary standards that originated in about the same way. Our foot-rule started out as the length of a man's foot. So, in the early days of history, the foot varied in length, sometimes as much as 3 or 4 inches. Once the ancients started using arms and feet for measuring distance, it was only natural that they also thought of using fingers, hands and legs. They also may have discovered that some surprising ratios existed in body measurements. What is now called an inch originally was the width of a man's thumb. It also was the length of the forefinger from the tip to the first joint. Twelve times that distance made a foot. Three times the length of the foot was the distance from the tip of a man's nose to the end of his outstretched arm. This distance very closely approximates what is called the yard. Two yards equaled a fathom which, thousands of years ago, was the distance across a man's outstretched arms. Half a yard was the 18-inch cubit, and half a cubit was called a span, which was the distance across the hand from the tip of the thumb to the tip of the little finger when the fingers were spread out as far as possible. A hand was half a span.

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Sometime after the Magna Charta was signed in the Thirteenth Century, King Edward I of England took a step forward. He ordered a permanent measuring stick made of iron to serve as a master standard yardstick for the entire kingdom. This master yardstick was called the "iron ulna", after the bone of the forearm, and it was standardized as the length of a yard, very close to the length of our present-day yard. King Edward realized that constancy and permanence were the key to any standard. He also decreed that the foot measure should be one-third the length of the yard, and the inch one thirty-sixth. King Edward II, in 1324, reverted back to the seed concept of the ancients and passed a statute that "three barleycorns, round and dry," make an inch.

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In 1672, Sir Isaac Newton presented the world with new ideas on the nature of light and color. He had noticed that when two flat pieces of glass were pressed together, he could see circular bands of rainbow-like colors. These were called Newton's Rings. Actually, Newton had come upon a very precise method of measurement, but he didn't recognize it as such at the time. Later, other scientists were to build on Newton's seminal findings and establish a new branch of science called interferometry . Today, this method of using a ray of light as a measuring stick enables man to measure distances within millionths of an inch or a millimetre.

As the scientists were experimenting in their laboratories, practical tradesmen were making themselves permanent standards. In 1793, during Napoleon's time, the French government adopted a new system of standards called the metric system, based on what they called the metre. The metre was supposed to be one tenth-millionth part of the distance from the North Pole to the Equator when measured on a straight line running along the surface of the earth through Paris. With the metre now determined as the basis of the metric system, other linear units of the system were set up in decimal ratios with the metre. With this system, all units are in multiples of ten: ten decimetres in a metre, a hundred centimetres in a metre, and a thousand millimetres in a metre. In the other direction, there are ten metres in a dekametre, a hundred metres in a hectometre, and a thousand metres in a kilometre. Compared to the yardstick, the metre is just a little longer: 39.37 inches long.

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While France was evolving the metric system, England also was setting up a more scientifically accurate determination of the yard. Where the French relied on the assumed constancy of the earth's size as a basis for the permanency of their standards, the British turned to the measured beat of the pendulum. Galileo already had learned the secrets of a pendulum. He found that the length of time it took for a pendulum to complete a swing depended upon the length of the pendulum itself. The longer the pendulum, the slower it swung. He also found that a pendulum a little over 39 inches long would swing through its arc in exactly one second. Since a pendulum always behaves exactly the same way under the same conditions, here was another unchanging distance upon which to base a standard measurement.

In 1824, the English Parliament legalized a new standard yard which had been made in 1760. It was a brass bar containing a gold button near each end. A dot was engraved in each of these two buttons. These two dots were spaced exactly 1 yard apart. The same act that legalized this bar as the standard for England also made the provision that, in the event it was lost or destroyed, it should be replaced using the pendulum method to determine its length. A few years copies of both the English yard and the French metre standards were brought to the United States. The English system of measuring was almost universally adopted in the United States.

The kilogram is the weight of some object they keep in France. If you were to steal it, the metric system would stop working, and then you could hold it for ransom for massive amounts of money. At least, that's how it works in my imagination.

The kilogram is the weight of some object they keep in France. If you were to steal it, the metric system would stop working, and then you could hold it for ransom for massive amounts of money. At least, that's how it works in my imagination.

There were many standards for measuring stuff to start with, most were very sketchy but over time they defined themselves.

The need for measuring quantities, weights, distances, etc... increased with time. The Eastern Meditereanean trading nations are at the roots of most measurements. The Romans used a system derived form the Greeks, the Greeks, used one derived from the Egyptions, they used the Mesapotonian system, and thats about as far back as we go.

Most measurements were made out of practicallity, how much land can be plowed in a day, how far can an army march, how much grain is in a soldiers ration, etc... of course accuracy was minimal but with universal sytems began to appear.